1. Field of the Invention
This invention relates to an anti-skid control apparatus for a vehicle braking system which can prevent locking of the wheels.
2. Description of the Prior Art
An anti-skid control apparatus for a vehicle braking system is known that includes a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, said fluid pressure control valve device receiving control signals of a control unit measuring the skid condition of the wheel to control the brake fluid pressure to the wheel cylinder; a hydraulic reservoir which, when the brake fluid pressure to the wheel cylinder is decreased with control of the fluid pressure control valve device, receives and stores the brake fluid discharged through the fluid pressure control valve device from the wheel cylinder; a pressure fluid supply conduit connecting the master cylinder with the fluid pressure control valve device; and a fluid pump for returning the brake fluid from the hydraulic reservoir into the pressure fluid supply conduit.
When the fluid pressure control valve device is provided for each of four wheels, and their fluid pressures are independently controlled, there is no problem on control operation. Or when the fluid pressure control valve device is provided for each of the front wheels, and for both of the rear wheels in common, there is no problem on control operation. In the latter case, the one common fluid pressure control valve device is controlled on the basis of the lower one of the speeds of the rear wheels.
However, in the above cases, three or four fluid pressure control valve devices are used. Accordingly, the whole anti-skid control apparatus is large-sized, and very heavy. Since the fluid pressure control valve device is expensive, the system is costly.
For example, it is considered that the brake fluid pressures of the front wheels are controlled by the two fluid pressure control valve devices respectively in the diagonal or X-type conduit system, and the brake fluid pressures of the rear wheels are controlled in common with the front wheels. However, when the vehicle runs on a road where the right and left sides are considerably different in frictional coefficient from each other, there is the fear that the one rear wheel being diagonal to the one front wheel on the higher frictional coefficient side is locked. In that case, the steering of the vehicle becomes unstable, and that is very dangerous.
Further, it is considered that proportioning valves are provided for the rear wheels, respectively. However, the brake fluid pressures of the rear wheels increases in proportion to the input fluid pressures to the proportioning valves. The fear of locking cannot be avoided.
Accordingly, in order to provide an anti-skid control apparatus for a vehicle braking system which can be small-sized and light, and can avoid the fear of locking of rear wheels, this applicant previously proposed an anti-skid control apparatus for a vehicle braking system which includes; a fluid pressure control valve device arranged between a master cylinder and a wheel cylinder of a brake for the wheel, said fluid pressure control valve device receiving control signals of a control unit measuring the skid condition of the wheel to control the brake fluid pressure to said wheel cylinder; a hydraulic reservoir which, when the brake fluid pressure to said wheel cylinder is decreased with control of said fluid pressure control valve device, reserves the brake fluid discharged through said fluid pressure control valve device from said wheel cylinder; a pressure fluid supply conduit connecting said master cylinder with said fluid pressure control valve device; and a fluid pump for returning the brake fluid from said hydraulic reservoir into said pressure fluid supply conduit; said fluid pressure control valve device being provided for a pair of front wheels, respectively, a valve apparatus receiving fluid pressures of wheel cylinders of said front wheels being arranged between said pair of front wheels and a pair of rear wheels, and when any one of said fluid pressure control valve devices starts to control, at least the fluid pressure of the one of said rear wheels, being at the same side as the one of said front wheels, the fluid pressure of the wheel cylinder of which is lower, is controlled in accordance with the lower one of the fluid pressures of the wheel cylinders of said front wheels by said valve apparatus U.S. Pat. No. 4,637,664. This assignee is the assignee of the following U.S. patent applications: Ser. Nos. 891,361, 903,498, 907,596, 916,406 and 57,802 for the anti-skid control apparatus using the above-described valve apparatus, in addition to the U.S. patents listed above.
With the above-described arrangements, both of the rear wheels can be securely prevented from locking. However, when the frictional coefficient of the road on which the vehicle is running is very high, the deceleration of the vehicle is rapidly increased with the brake. Accordingly, the vehicle or automobile sinks or falls forward. The forward falling of the automobile is the so called "nose-dive phenomenon". The falling or swing of the automobile occurs due to the inertia of the center of the gravity of the automobile, when the wheels are braked. The load distributions to the front and rear wheels change due to the rotational force around the contact points between the front or rear wheel and the ground. In that case, the load to the rear wheels becomes very small. Accordingly, although the pressure-reducing proportioning valves are arranged between the master cylinder and the rear wheels, the rear wheels tend to lock sooner than the front wheels. In order to prevent the locking of the rear wheels or rear wheel, the fluid pressure control valve device or devices are controlled to decrease the brake pressure. Since the pressure-reducing proportioning valves are arranged, the brake pressure to the front wheel should be decreased very much. The required braking force to the front wheel is much reduced. As the result, the braking distance of the automobile becomes very long.
When the frictional coefficients .mu. are different at the sides of the road, or for example, when the frictional coefficient .mu. of the left side is extremely high, and that of the right side is high, the one of the rear wheels running on the right side of the road tends to lock sooner than the other rear wheel. The brake pressure to the one rear wheel is decreased to prevent that wheel from locking. However, the brake pressure to the front wheel is decreased and the brake pressure to the wheels running on the higher .mu. side is increased. As the result, a yaw moment or torque is applied around the center of the gravity of the automobile. The automobile yaws. That is very dangerous.